Ehhh...I don't know. If they are exactly what you're looking for, and there is free shipping and free returns! That's tough.
I know ASOS has a lot of horror stories, but I have personally never experienced any. I'm sure for every horror story they have, there are at least 50 transactions that go perfectly smoothly. At least.
I guess I am biased because I did this recently with a pair of Camper sandals. Like you, I wasn't sure about the whole sizes, but the 36 was sold out and there was only one 37 left. I was mentally preparing myself to be disappointed and end up returning them..but to my surprise, they ended up fitting. They were exactly what I wanted, and they were on sale...it was just too good an opportunity to pass up, and I'm glad I didn't. I guess in my case it was an easier decision since it was Zappos and I didn't even have to think twice about the customer service.